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按照传统的观点,在地壳范围内发生的大多数地震属于断层地震,因为在地壳范围内缺乏相变的条件。对于深源地震则认为它是一种剪切运动加上体积的收缩或扩张的结果。然而,近几年来我们在整理西海固地区震源机制资料的过程中发现,这个地区有一些地震的P波初动是划不出节线的,有的以正号波为主,有的则以负号波为主。其震源深度大都位于地壳范围内。这类地震的直接成因显然是不能用一种简单的断层错动来解释的。本文列举了西海固地区这几次特殊P波初动分布的资料,并对形成这类地震的成因进行了初步探讨。
Traditionally, most of the earthquakes that occurred in the crust range are fault earthquakes because of the lack of conditions for phase change within the crust. For deep-seated earthquakes it is considered as a result of a shearing movement plus volume contraction or expansion. However, in the past few years, during the process of compiling source mechanism information in the Xihaigu area, we found that there are some earthquakes in the area where the first P wave can not be plotted. Some are dominated by positive waves, while others are based on Negative wave based. Most of the focal depths are in the crust. The immediate cause of such earthquakes is obviously not explained by a simple fault slip. This paper lists the data of the initial P wave distributions in the Xihaigu area and discusses the causes of the formation of such earthquakes.